If a mail server receives a
request to deliver a message to a non-existent user, then it will reply with a
5XX series error.

The e-mail client or
downstream mail server attempting the send operation will receive this error and
will generate a non-delivery message to the user.

The cause of the error is
either that the user/mailbox does not exist (and has been specified incorrectly
when the user sent the message) or that a server configuration problem is
stopping the server from resolving the users e-mail address.

SYMPTOMS

When sending messages
through MailEnable the following error is returned in the
client:

550 Requested action not
taken: mailbox unavailable or not local

Mailboxes are unable to
send and receive.

This article only applies
to servers running the Plesk Control Panel and can occur with all mailboxes
on the system or only one/some.

CAUSE

The Plesk control system
has the ability to disable mailboxes within its interface. To do this the
panel edits the MailEnable configuration file (address-map.tab) and adds a !
into the mailbox address which makes the mailbox unrecognisable by the
MailEnable services.

To verify this is the case,
check in the MailEnable Administration program for a mailbox that is
experiencing the issue and see if the mailbox is disabled. A mailbox
that is disabled within MailEnable will have a red cross on the mailbox
icon.

For more advanced users, or
to verify if Plesk was the service that disabled the mailbox, open the following
configuration file:

MailEnable\Config\Address-map.tab

and search for any
mailboxes that appear like the following entry:

[!SMTP:email@example.com] a
normal mailbox would appear without the ! before the SMTP: part of the
configuration string.

RESOLUTION

To resolve this issue,
re-enable all the mailboxes within the Plesk control panel, or search through
the address-map.tab file and remove all ! characters.